Soil mixing techniques whereby drilled ground is mixed in situ with a hydraulic binder are nowadays commonly used for improving substructures. The tools used generally employ special equipment resembling augers that are caused to rotate about a vertical axis. Those machines enable rectangular wall elements to be made by juxtaposing a plurality of augers, thereby requiring high-power machines to be used whenever the trench needs to reach depths greater than 10 meters (m).
A new type of machine has been in existence for several years that makes it possible to make rectangular foundation elements out of soil cement, i.e. by mixing a hydraulic binder with the soil that has been dug so as to make a portion of a trench, while also mixing the mixture. This operation is referred in the present patent application by the term “digging a trench while mixing cuttings with another material”.
Naturally, the mixture must be left in place in the trench that is being made so as to end up with a wall in the soil that results from the mixture of cut soil and hydraulic binder setting, which wall has its shape defined by the shape of the trench.
A machine of this type is described for example in patent applications US 2005/0000123 and US 2004/0234345.
That machine is constituted essentially by two pairs of cutters mounted on a support structure. Each pair of cutters is connected to a hydraulic motor. The motors are housed in a relatively bulky box located above the cutters.
When the motor is mounted in a bulky box, the drawback presented by the machine consists in the box in which the motors are housed presenting a relatively large apparent area. The presence of this box of large dimensions interferes considerably with raising the tool after it has performed the mixing, since the box needs to “barge through” the mixed material constituted by soil cuttings and hydraulic binder. In some circumstances, while the machine is being raised, the presence of this box can lead to the machine becoming blocked in the panel filled with the mixture constituted by the drilling cuttings and the hydraulic binder.
In the machine of that type, that is described in patent application US 2005/0229440, the two pairs of cutters are connected by a common transmission to a single motor that may be situated above the surface of the ground. The transmission is then complex and its efficiency mediocre.
Furthermore, since the two pairs of cutters are driven by the same motor, all of the cutters rotate at the same speed. Unfortunately, it can sometimes be advantageous to be able to give each pair of cutters a different speed of rotation, in particular to correct departures from the vertical while digging the trench. In addition, the power from the motor is shared between the two pairs of cutters providing operation is normal. However, if one pair of cutters becomes blocked, then all of the power from the motor must be absorbed by the other pair of cutters. That requires the system to be dimensioned mechanically so as to be able to accommodate this situation.
Excavator machines are also known for making trenches in the soil. Such machines are usually constituted by two pairs of rotary cutters mounted at the bottom end of a structure of large dimensions. The top end of the structure is secured to support means that are generally constituted by cables.
In horizontal section, the structure of the machine is generally rectangular in shape with dimensions substantially equal to the overall dimensions of the pairs of cutters. Thus, the dimensions of the right section of the structure are substantially equal to the dimensions of the horizontal section of the portion of trench that the machine can dig as it moves downwards.
Thus, the walls of the structure are substantially in contact with the walls of the portion of trench being dug, thereby ensuring that the machine is guided vertically in order to obtain a portion of trench that is likewise substantially vertical.
In addition, the soil cut by the cutters is removed via a suction tube having its inlet disposed between the walls of the cutters beneath the structure.
It is clear that such an excavator machine is totally incapable of mixing the cut soil with the hydraulic binder, so that the mixture is left in place in the portion of trench being dug in order to make the wall in the soil.
Documents EP 0 262 050 and GB 1 430 617 describe such a machine.